doris writer default value error

1. give a default value 'false' for the column __DELETE_SIGN__,
2. use the datax to migrate data from maxcompute to doris
3. check the value of __DELETE_SIGN__

expect result:
the value of __DELETE_SIGN__ is 'false'

actual result:
the value of __DELETE_SIGN__ is null

Reproduce the DataX migration from MaxCompute to Doris described in the issue, focusing on the __DELETE_SIGN__ column. Check the migrated value against the expected false value rather than null. Done means the same migration produces false for __DELETE_SIGN__.

Written by the indexing model from the issue text.